Easy, free fix: slide the frog off the scabbard and turn it around. This won't work with all models and probably won't be pretty but it sometimes does the job.
More difficult, more expensive fix: fabricate a new scabbard.
Easy, costly fix: pay someone else to fabricate a new scabbard.
Extremely difficult, free fix: learn to be a righty.
I'd opt for the first one myself before I did anything. It usually works, it costs nothing, and it takes five seconds to reverse if it's not for you. Just be sure that the frog isn't nailed to the scabbard first.